Optical disk and ultraviolet-curing composition for optical disk

ABSTRACT

An object is to provide a technique of imparting durability which is equal to that of existing types of DVD-5 and DVD-10 as well as DVD-9 using gold or a silicon based compound as a semitransparent film, to a DVD obtained by bonding two substrates having information recording layers, respectively, on each outermost layer of which a thin film of silver or an alloy composed primarily of silver is formed. Using an optical disc wherein ratios of error rates before and after an environmental test at 80° C. and 85% RH for 96 hours is 10 or less, and an ultraviolet curable composition for optical disc wherein a water absorption rate of the cured coating film is 2.0 wt % or less and a water-vapor permeability at a film thickness of 50 μm is 70 g/m 2 ·day or less, the object described above is achieved.

TECHNICAL FIELD

The present invention relates to a bonded optical disc, and moreparticularly, relates to a bonded optical disc containing silver or analloy primarily composed of silver in an information recording layer,and to an ultraviolet curable composition which is suitable forproduction of a bonded optical disc.

BACKGROUND ART

DVDs (“digital video discs” or “digital versatile discs”) are producedby a method in which two discs, of which at least one disc has aninformation recording layer, are bonded together, and an ultravioletcurable composition is generally used as an adhesive.

In the case of read-only DVDs, the structure of the two bondedsubstrates may be of one of the following types or the like, which canbe selected properly depending on the application: a type (DVD-10) inwhich polycarbonate substrates, are used which are each provided on onesurface thereof with irregularities, called “pits”, which correspond torecorded information and further, as a reflecting film for a laser beamfor reading information, an aluminum layer, for example, is formed, toprovide an information recording layer; a type (DVD-5) in which, as oneof the substrates, a transparent polycarbonate substrate having noinformation recording layer is used; a type (DVD-9) in which one of suchsubstrates is formed with a semitransparent film of a metal composedprimarily of gold, a silicon compound, or the like to provide aninformation recording layer; and a type (DVD-18) in which each substratehas two information recording layers on one surface, and two suchsubstrates are bonded together.

The semitransparent film of DVD-9 is primarily composed of gold, asilicon compound or the like. However, gold has a drawback in that it isdisadvantageous in cost because of its very high price, while thesilicon compound has a drawback in that formation of a film is verydifficult.

On the other hand, silver or an alloy composed primarily of silver issuitable for use as a semitransparent film for DVD-9 because it has alower price compared to gold and can be easily formed into a film.However, since silver or an alloy composed primarily of silver ischemically unstable, when using a conventional ultraviolet curableadhesive for DVDs, it causes deterioration of the surface when left inhigh temperature and high humidity environments for a long time, thusleading to errors in reading recorded information, poor appearance orthe like.

Japanese Patent Application, First Publication No. Hei 9-11626 disclosesthat a fluororesin coating having a low water absorption rate is formedas a protective layer for silver or an alloy composed primarily ofsilver. However, the fluororesin coating has poor adhesion and is notultraviolet-curable, thus making it difficult to use it as it is in acurrent DVD production process using an ultraviolet curable adhesive.Therefore, the fluororesin coating is not suitable for practical use.

As described above, although silver or an alloy composed primarily ofsilver is quite suitable for use in a semitransparent film for DVDs, anultraviolet curable composition having sufficient protectivecharacteristics has not yet been developed, and therefore, DVDs usingsilver or an alloy composed primarily of silver have not yet beencommercialized.

DISCLOSURE OF THE INVENTION

An object to be achieved by the present invention is to provide DVDshaving durability equal to that of existing DVDs of the DVD-5 and DVD-10types as well as those of the DVD-9 type using gold or a silicon basedcompound as a semitransparent film, by using the above-described discwherein a thin film of silver or an alloy composed primarily of silveris formed, and to provide an adhesive composition which does not causedeterioration of the thin film of silver or an alloy composed primarilyof silver.

The present inventors have carried out intensive research to solve theabove problems, and as a result, have found that durability equal tothat of a conventional optical disc using gold or a silicon basedcompound can be realized by observing the increase in the error rateafter an environmental test in a bonded optical disc using a thin filmof silver or an alloy composed primarily of silver as a semitransparentfilm in an outermost layer of an information recording layer, andsetting the error rate in a specified range, and that a remarkableeffect can be obtained by setting a water absorption rate and awater-vapor permeability of a cured coating, and thus the presentinventors have achieved the present invention.

That is, the present invention provides an optical disc comprising twosubstrates having information recording layers, respectively, a thinfilm of silver or an alloy composed primarily of silver being formed onan outermost layer of the information recording layer of at least onesubstrate, said two substrates being bonded together by an ultravioletcurable composition, wherein the optical disc is characterized in thatratios of error rates before and after an environmental test at 80° C.and 85% RH for 96 hours are 10 or less.

As the substrate used in the optical disc of the present invention,those typically used as a substrate for optical disc can be used and,particularly preferably, a polycarbonate substrate can be used.

The “alloy composed primarily of silver” used in the optical disc of thepresent invention includes, for example, silver incorporated withseveral % of palladium, copper, or the like therein.

As used herein, ratios of error rates are defined as ratios of PI errorrates before and after an environmental test (error rate after theenvironmental test/error rate before the environmental test). As usedherein, the term “error rates” has the same meaning as that used bythose skill in the art in the case of evaluating an optical disc such asa DVD or the like.

The environmental test is conducted by allowing an optical disc toremain in an environment of 80° C. and 85% RH for 96 hours. In thisenvironmental test, an apparatus such as a thermo-hygrostat or the likecan be used.

The optical disc of the present invention is preferably a DVD (“digitalvideo disc” or “digital versatile disc”), for example, a read-only DVDsuch as one of the DVD-5, DVD-10, DVD-9 or DVD-18 type, a writable DVDsuch as a DVD-R, or a rewritable DVD such as a DVD-RAM, DVD-RW, orDVD+RW, and the optical disc is more preferably a DVD-9. The opticaldisc of the present invention is not limited to these DVDs and may be,for example, an optical disc in which a cover layer having a thicknessof about 0.1 mm is formed by bonding a transparent film or coating anultraviolet curable composition on a substrate having a thickness ofabout 1.1 mm, an optical disc having a structure comprising threesubstrates having a thickness of 0.6 mm or less bonded to each other orthe like.

If there is no change in the error rate before and after theenvironmental test, PI error rate ratios before and after theenvironmental test are 1. In general, since the error rates aredetermined by characteristics of each disc substrate, the error ratesvary considerably, thus making it difficult to set to a fixed value fora bonded type disc. Such a relative ratio makes possible discussion on auniform basis.

Furthermore, the present invention provides an ultraviolet curablecomposition for an optical disc, which is used in the production of anoptical disc comprising two substrates having information recordinglayers, respectively, a thin film of silver or an alloy composedprimarily of silver being formed on an outermost layer of theinformation recording layer of at least one substrate, said twosubstrates being bonded together by an ultraviolet curable composition,wherein the ultraviolet curable composition is characterized in that awater absorption rate of the cured coating film is. 2.0 wt % or less anda water-vapor permeability at a film thickness of 50 μm is 70 g/m²·dayor less. Preferably, the ultraviolet curable composition is composedprimarily of an ultraviolet curable compound composed of a(meth)acrylate having a alicyclic structure, and a photopolymerizationinitiator.

The ultraviolet curable composition for an optical disc of the presentinvention may be used as an adhesive of a bonded disc and may be used asa protective coating agent for a thin film of an alloy of silver or analloy composed primarily of silver. In any case, an optical disc havingan excellent durability can be obtained.

As the ultraviolet curable composition used in the ultraviolet curablecomposition of the present invention, monofunctional (meth)acrylate andpolyfunctional (meth)acrylate can be used as a polymerizable monomercomponent. These may be used singly, or two or more thereof may be usedin combination. In the present invention, acrylate and methacrylatetogether are called (meth)acrylate.

The polymerizable monomer which can be used in the present inventionincludes, for example, the following.

The monofunctional (meth)acrylate includes, for example, (meth)acrylateand the like, having a group such as methyl, ethyl, propyl, butyl, amyl,2-ethylhexyl, octyl, nonyl, dodecyl, hexadecyl, octadecyl, cyclohexyl,benzyl, methoxyethyl, butoxyethyl, phenoxyethyl, nonylphenoxyethyl,tetrahydrofurfuryl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl,3-chloro-2-hydroxypropyl, dimethylaminoethyl, diethylaminoethyl,nonylphenoxyethyltetrahydrofurfuryl, caprolactone-modifiedtetrahydrofurfuryl, isobornyl, dicyclopentanyl, dicyclopentenyl,dicyclopentenyloxyethyl or a similar group as a substituent group.

In addition, the polyfunctional (meth)acrylate includes, for example,di(meth)acrylates of 1,3-butylene glycol, 1,4-butanediol,1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentylglycol, 1,8-octanediol, 1,9-nonanediol, tricyclodecanedimethanol,ethylene glycol, polyethylene glycol, propylene glycol, dipropyleneglycol, tripropylene glycol, polypropylene glycol, etc.,di(meth)acrylate of tris(2-hydroxyethyl)isocyanurate, di(meth)acrylateof a diol obtained by adding 4 moles or more of ethylene oxide orpropylene oxide to 1 mole of neopentyl glycol, di(meth)acrylate of adiol obtained by adding 2 moles of ethylene oxide or propylene oxide to1 mole of bisphenol A, di- or tri(meth)acrylate of a triol obtained byadding 3 moles or more of ethylene oxide or propylene oxide to 1 more oftrimethylolpropane, di(meth)acrylate of a diol obtained by adding 4moles or more of ethylene oxide or propylene oxide to 1 more ofbisphenol A, trimethylolpropane tri(meth)acrylate, pentaerythritoltri(meth)acrylate, dipentaerythritol poly(meth)acrylate, ethyleneoxide-modified alkylated phosphoric acid (meth)acrylate, etc.

To control the water absorption rate and water-vapor permeability,(meth)acrylates having an alicyclic substituent such as cyclohexyl,isobornyl, dicyclopentanyl, dicyclopentenyl, dicyclopentenyloxyethyl,tricyclodecanedimethanol, etc., among the above substituents arepreferably used.

Further, those which can be used in combination similarly to thepolymerizable monomer include as polymerizable oligomers, polyester(meth)acrylate, polyether (meth)acrylate, epoxy (meth)acrylate, urethane(meth)acrylate, etc.

In the present invention, the photopolymerization initiator may be anyknown, commonly used one with which ultraviolet curable compoundsrepresented by the polymerizable monomers and/or polymerizable oligomersused can be cured. As the photopolymerization initiator, moleculecleaving type or hydrogen eliminating type ones are suitable for thepresent invention.

The photopolymerization initiators preferably used in the presentinvention include benzoin isobutyl ether, 2,4-diethylthioxanthone,2-isopropylthioxanthone, benzyl, 2,4,6-trimethylbenzoyldiphenylphosphineoxide, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, etc.,and further, as molecule cleaving type ones other than these,1-hydroxycyclohexyl phenyl ketone, benzoin ethyl ether, benzyl dimethylketal, 2-hydroxy-2-mehtyl-1-phenyl-propan-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one and2-methyl-1-(4-methylthiophenyl)-2-morpholino-propan-1-one, etc., may beused in combination. Further, benzophenone, 4-phenylbenzophenone,isophthalophenone, 4-benzoyl-4′-methyl-diphenyl sulfide, etc., which arehydrogen eliminating type photopolymerization initiators, may be used incombination.

In combination with the above photopolymerization initiator can be used,as a sensitizer, amines that do not undergo addition polymerizationreaction with the above-described polymerizable components, such as, forexample, triethylamine, methyldiethanolamine, triethanolamine,p-diethylaminoacetophenone, ethyl p-dimethylaminobenzoate, isoamylp-dimethylaminobenzoate, N,N-dimethylbenzylamine, and4,4′-bis(diethylamino)benzophenone. Of course, it is preferred to selectfor use the above-described photopolymerization initiators andsensitizers from those which are superior in solubility in the curablecomponents and which do not interfere with ultraviolet lighttransmission.

It is preferred to use ultraviolet curable compositions which are liquidat room temperature to 40° C. It is preferred not to use solvents. Ifsolvents are used, it is preferred to limit the amount to a level assmall as possible. In the case where coating of the above-describedcomposition is performed using a spin coater, it is preferred to adjustthe viscosity to 20 to 1,000 mPa·s and in the case in which a relativelythick film is to be obtained, it may be adjusted to 100 to 1,000 mPa·s.

The composition of the present invention may, if necessary, have blendedtherein other additives such as thermal polymerization inhibitors,antioxidants represented by hindered phenols, hindered amines,phosphites, etc., plasticizers, silane coupling agents represented byepoxysilane, mercaptosilane, (meth)acrylsilane, etc., and the like inorder to improve various properties. These are selected for use fromthose which are superior in solubility in curable components, and whichdo not interfere with ultraviolet transmission.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail basedon preferred embodiments of the present invention with reference to theDVD-9 type.

First, one disc-like plastic substrate in which a 40-60 nm metal thinfilm is sputtered on irregularities called “pits”, corresponding to therecorded information, and one disc-like plastic substrate in which a10-30 nm semitransparent thin film of gold is sputtered onirregularities called “pits”, corresponding to the recorded information,are prepared.

Then, an ultraviolet curable composition is prepared such that a waterabsorption rate of the cured coating film is 2.0 wt % or less and awater-vapor permeability at a film thickness of 50 μm is 70 g/m²·day orless, using two kinds or more of polyfunctional (meth)acrylate havingtwo or more (meth)acryloyl groups as a polymerizable monomer orpolymerizable oligomer, and optionally a monofunctional monomer, asneeded, and further using photopolymerization initiators in an amount of2 to 7 parts by weight per 100 parts by weight of the ultravioletcurable composition. Since the water absorption rate and water-vaporpermeability are easily controlled, a (meth)acrylate having an alicyclicstructure is preferably used as the polymerizable monomer orpolymerizable oligomer.

The above-described composition is coated on the metal thin film surfaceof a 40-60 nm metal thin film-sputtered disc-like plastic substrate anda 10-30 nm silver or silver alloy semitransparent film-sputtereddisc-like plastic substrate is bonded such that the semitransparent filmsurface is opposed to the metal thin film surface. Ultraviolet light isirradiated from one side or both sides of the bonded disc to unite thetwo to form a DVD-9 disc.

Irradiation with ultraviolet light can be performed by a continuouslight irradiation system using, for example, metal halide lamp, mediumpressure mercury lamp, etc., and can also be performed by a flashirradiation system described in U.S. Pat. No. 5,904,795. The flashirradiation system is particularly preferred in that curing can beperformed efficiently.

The adhesive layer of the DVD-9 thus obtained is composed of a curedcoating film wherein the water absorption rate of the cured coating filmis 2.0 wt % or less and the water-vapor permeability at a film thicknessof 50 μm is 70 g/m²·day or less, and wherein the ratios of error ratesbefore and after an environmental test at 80° C. and 85% RH for 96 hoursis 10 or less. This optical disc does not undergo any change inappearance even after storage for a long time and has durability whichis equal to that of a conventional DVD-9 using gold or a silicon basedcompound.

The same durability can be obtained even if the DVD-9, wherein asemitransparent film of silver or a silver alloy is protectively coatedwith the above ultraviolet curable composition, is produced by bondingdisc substrates together using a freely selected ultraviolet curablecomposition. The protective coating can be obtained, for example, bycoating an ultraviolet curable composition in a thickness of about 10 μmusing a spin coater, and curing the ultraviolet curable composition byirradiation with ultraviolet light. In this case, the ultravioletcurable composition is preferably prepared such that the viscosity is 20to 300 mPa·s. Irradiation with ultraviolet light can be performed by acontinuous light irradiation system using, for example, a metal halidelamp, medium pressure mercury lamp, etc., in the same manner as in thecase of bonding, and can also be performed by a flash irradiationsystem. The continuous light irradiation system is preferred because anexisting protective coating apparatus for CDs can be used.

EXAMPLES

Next, the present invention will be described in detail by examples.However, the present invention is not limited to these examples. In theexamples, “parts” indicates “parts by weight”.

Example 1

Twenty six (26) parts UNIDIC V-5500 (manufactured by DAINIPPON INK ANDCHEMICALS, INC.) as bisphenol A type epoxyacrylate, 30 parts oftricyclodecanedimethylol diacrylate, 10 parts of bisphenol A typeethylene oxide-modified diacrylate, 15 parts of tripropylene glycoldiacrylate, 11 parts of isobornyl acrylate, 1.5 parts of ethyleneoxide-modified trimethylolpropane triacrylate, 0.2 parts of ethyleneoxide-modified phosphoric acid methacrylate, 0.3 parts of ethyldimethylaminobenzoate, and 2 parts of2,4,6-trimethylbenzoyldiphenylphosphine oxide and 4 parts of1-hydroxycyclophenyl ketone as a photopolymerization initiator weremixed and dissolved with heating at 60° C. for one hour to prepare apale yellow transparent ultraviolet curable composition.

Example 2

Eighteen (18) parts of FAU-74-2 (manufactured by DAINIPPON INK ANDCHEMICALS, INC.) as urethane acrylate, 77 parts oftricyclodecanedimethylol diacrylate, 4.5 parts of trimethylolpropanetriacrylate, 0.2 parts of ethylene oxide-modified phosphoric acidmethacrylate, 0.3 parts of ethyl dimethylaminobenzoate, and 5 parts of1-hydroxycyclophenyl ketone as a photopolymerization initiator weremixed and dissolved with heating at 60° C. for one hour to prepare apale yellow transparent ultraviolet curable composition.

Comparative Example 1

Twenty (20) parts of FAU-504 (manufactured by DAINIPPON INK ANDCHEMICALS, INC.) as urethane acrylate, 21 parts of UNIDIC V-5500(manufactured by DAINIPPON INK AND CHEMICALS, INC.) as bisphenol A typeepoxyacrylate, 13 parts of tripropylene glycol diacrylate, 10 parts oftetrahydrofurfuryl acrylate, 17 parts of 2-hydroxy-3-phenoxypropylacrylate, 11 parts of ethylcarbitol acrylate, 1.5 parts of ethyleneoxide-modified trimethylolpropane triacrylate, 0.2 parts of ethyleneoxide-modified phosphoric acid methacrylate, 0.3 parts of ethyldimethylaminobenzoate, and 2 parts of2,4,6-trimethylbenzoyldiphenylphosphine oxide and 4 parts of1-hydroxycyclophenyl ketone as a photopolymerization initiator weremixed and dissolved with heating at 60° C. for one hour to prepare apale yellow transparent ultraviolet curable composition.

Comparative Example 2

Twenty (20) parts of DICLITE UE-8200 (manufactured by DAINIPPON INK ANDCHEMICALS, INC.) as bisphenol A type epoxyacrylate, 34.5 parts oftrimethylolpropane triacrylate, 19 parts of bisphenol A type ethyleneoxide-modified diacrylate, 10 parts of tripropylene glycol diacrylate,11 parts of neopentylglycol diacrylate, 0.2 parts of ethyleneoxide-modified phosphoric acid methacrylate, 0.3 parts of ethyldimethylaminobenzoate, and 2 parts of2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2 parts ofbenzophenone and 1 part of dimethylaminoacetophenone as aphotopolymerization initiator were mixed and dissolved with heating at60° C. for one hour to prepare a pale yellow transparent ultravioletcurable composition.

(Evaluation of Characteristics)

Using the compositions obtained in the Examples and Comparative Examplesdescribed above, the water absorption rate, the water-vapor permeabilityand the ratios of error rates of the DVD-9 bonded discs as well as theappearance after the environmental test were evaluated by the followingtest methods 1, 2, and 3. The results are summarized in Table 1.

(Test Method 1: Measurement of Water Absorption Rate of Cured CoatingFilm)

The ultraviolet curable composition was coated on a glass plate to afilm thickness of about 100 μm. Then, under a nitrogen atmosphere, usingas a UV lamp an M03-L31 (120 W/cm metal halide lamp, with a cold mirror)manufactured by EYE GRAPHICS CO., LTD., irradiation of 0.5 J/cm² (anultraviolet actinometer: UVPF-36, manufactured by EYE GRAPHICS CO.,LTD.) at a lamp height of 10 cm was carried out to prepare a curedcoating film. The water absorption rate of the cured coating film wasmeasured according to the Method B of JIS K7209-1984.

(Test Method 2: Measurement of Water-vapor Permeability of Cured CoatingFilm)

The ultraviolet curable composition was interposed between two glassplates to a film thickness of about 100 μm using a spacer. Then, under anitrogen atmosphere, using as a UV lamp an M03-L31 (120 W/cm metalhalide lamp, with a cold mirror) manufactured by EYE GRAPHICS CO., LTD.,irradiation of 1.2 J/cm² (an ultraviolet actinometer: UVPF-36,manufactured by EYE GRAPHICS CO., LTD.) at a lamp height of 10 cm wascarried out to prepare a cured coating film, and test pieces (10 cmsquare) having a uniform thickness were made by peeling off from theglass plate.

The water-vapor permeability of the cured coating film was measured byusing a LYSSY type automatic water-vapor permeability tester L-80. Sincean actual thickness of the adhesive layer of the DVD is about 50 μm, avalue adjusted in terms of 50 μm was taken as the value of thewater-vapor permeability.

(Test Method 3: Environmental Test on DVD-9 Bonded Disc)

On a polycarbonate disc on which pits for recording information areformed and aluminum is sputtered to 50 nm, the above-describedcomposition was coated using a dispenser, and this polycarbonate discwas overlaid with a polycarbonate disc on which an alloy composedprimarily of silver is sputtered as a semitransparent film. Then, usinga spin coater, the discs were rotated so that the film thickness of thecured coating film became about 50-60 μm. Then, using a xenon flashirradiation apparatus (SBC-04 manufactured by Ushio Inc.), 10 shots ofirradiation with ultraviolet light were performed at a programmedvoltage of 1800 V in air from the side of the substrate with the silveralloy semitransparent film to produce a DVD-9 type bonded disc.

Environmental tests were conducted in which the bonded disc was left ina thermo-hygrostat (PR-2KP) manufactured by TABAI ESPEC CORP. under ahigh temperature and high humidity environment of 80° C. and 95% RH for96 hours and evaluation of the signal properties before and after theenvironmental test (measurements of error rates) were performed by usingan apparatus (SDP-1000) manufactured by PULSTEC INDUSTRIAL CO., LTD. Theratios of error rates before and after the test (after the environmentaltest/before the environmental test) were determined. The maximum valueof error rates was taken as the measured value. The appearance of thedisc after the environmental test was visually observed and the state ofthe silver alloy semitransparent film was evaluated.

TABLE 1 Comp. Comp. Example 1 Example 2 Example 1 Example 2 Waterabsorption of 1.5 0.9 3.0 2.1 cured coating film (wt %) Water-vaporpermeability 30 46 700 75 (value reduced in terms of 50 (m, g/m² · day)Ratios of error rates 6 2 impossible impossible to measure to measureEvaluation results of good good blackened blackened visual observation

As is apparent from Table 1, the bonded discs using the compositions ofExamples 1 and 2 as the adhesive layer, in which the water-vaporpermeability of the cured coating film is 2.0 wt % or less and thewater-vapor permeability at a film thickness of 50 μm is 70 g/m²·day orless, exhibited the ratios of error rates of 10 or less and hadexcellent durability because there was no change in appearance, as aresult of the environmental test of DVD-9 using the silver alloysemitransparent film.

On the other hand, with respect to DVD-9 using the compositions obtainedin Comparative Examples 1 and 2, the silver alloy film blackened, as aresult of the environmental test, and the state of the film wasdeteriorated such that normal signal detection cannot be performed andalso the measurement of the error rates cannot be performed.

Application Example

The composition of Example 1 was coated on a silver alloysemitransparent film to a film thickness of about 10 μm. Then, using asa UV lamp an M03-L31 (120 W/cm metal halide lamp, with a cold mirror)manufactured by EYE GRAPHICS CO., LTD., irradiation of 0.5 J/cm² at alamp height of 10 cm was carried out to form a protective coating. Usingthis substrate, DVD-9 bonded with the composition of Comparative Example1 was made by the test method 3. The ratio of error rate of the bondeddisc was 6 and exhibited excellent durability because of no change ofthe appearance.

INDUSTRIAL APPLICABILITY

It becomes possible to produce a DVD-9 in which deterioration of silveror an alloy composed primarily of silver is prevented and which also hasexcellent durability of the bonded disc after the high temperature andhigh humidity environmental test and very high reliability, by using thecomposition of the present invention as the adhesive layer or theprotective coating. A disc having very high reliability can also beproduced when using those in which a thin film of silver or an alloycomposed primarily of silver is formed on a reflective film of anoutermost layer of an information recording layer in a writable DVD-R, arewritable DVD-RAM, DVD-RW, DVD+RW or the like.

What is claimed is:
 1. An optical disc comprising two substrates havinginformation recording layers, respectively, a thin film of silver or analloy composed primarily of silver being formed on an outermost layer ofthe information recording layer of at least one substrate, said twosubstrates being bonded together by an ultraviolet curable composition,wherein the optical disc is characterized in that ratios of error ratesbefore and after an environmental test at 80° C. and 85% RH for 96 hoursis or less.
 2. The optical disc as claimed in claim 1, wherein said twosubstrates are bonded together by an ultraviolet curable composition inwhich a water absorption rate of the cured coating film is 2.0 wt % orless and a water-vapor permeability at a film thickness of 50 μm is 70g/m²·day or less.
 3. The optical disc as claimed in claim 2, wherein theultraviolet curable composition is composed primarily of an ultravioletcurable compound composed of a (meth)acrylate having a alicyclicstructure, and a photopolymerization initiator.
 4. The optical disc asclaimed in claim 1, wherein the thin film of silver or an alloy composedprimarily of silver is coated protectively with an ultraviolet curablecomposition in which the water absorption rate of the cured coating filmis 2.0 wt % or less and the water-vapor permeability at a film thicknessof 50 μm is 70 g/m²·day or less.
 5. The optical disc as claimed in claim4, wherein the ultraviolet curable composition, with which the thin filmof silver or an alloy composed primarily of silver is coated, iscomposed primarily of an ultraviolet curable compound composed of a(meth)acrylate having a alicyclic structure, and a photopolymerizationinitiator.
 6. The optical disc as claimed in claim 1, which is aread-only DVD.
 7. The optical disc as claimed in claim 1, which is arewritable or writable DVD.
 8. An ultraviolet curable composition foroptical disc, which is used in the production of an optical disccomprising two substrates having information recording layers,respectively, a thin film of silver or an alloy composed primarily ofsilver being formed on an outermost layer of the information recordinglayer of at least one substrate, said two substrates being bondedtogether by an ultraviolet curable composition, wherein the ultravioletcurable composition is characterized in that a water absorption rate ofthe cured coating film is 2.0 wt % or less and a water-vaporpermeability at a film thickness of 50 μm is 70 g/m²·day or less.
 9. Theultraviolet curable composition for optical disc as claimed in claim 8,which is composed primarily of an ultraviolet curable compound composedof a (meth)acrylate having a alicyclic structure, and aphotopolymerization initiator.